1. Field of the Invention
The present invention relates to a spindle motor and a bearing assembly for use in office automation equipment such as a computer and peripheral equipment thereof as a driving device/component for the rotating mechanism thereof, specifically to the spindle motor and the bearing assembly to enhance the run-out accuracy/nonrepeatable runout (NRRO) of a motor, and reliability, low noise, acoustic life, and rigidity, etc.
2. Description of the Prior Art
Spindle motors for driving a magnetic disk, e.g., a hard disk drive as a peripheral device of a computer, are classified broadly into two types in terms of the structure: the fixed shaft type in which a fixed shaft is installed upright on a base, and a rotor hub is supported to freely rotate on the fixed shaft through a bearing interposed between the fixed shaft and the rotor hub; and the rotary shaft type in which a rotary shaft is vertically installed on a rotor hub, and the rotary shaft is supported to freely rotate on a base through a bearing interposed between the rotary shaft and the base.
Generally, the fixed shaft type is provided with, as shown in FIG. 9, a base (flange) 02, a fixed shaft 010 that is installed upright on the base 02, a rotor hub (hub member) 03 that rotates relative to the base 02, and a bearing means 04 interposed between the fixed shaft 010 and the rotor hub 03. A recording medium such as a magnetic disk (not shown) is mounted on the rotor hub 03. A stator 015 is installed on the outer peripheral surface of an inner cylindrical wall 014 of the base 02, and a permanent magnet 016 is installed on the inner peripheral surface of an outer circumferential wall 013 of the rotor hub 03 so as to face the outer peripheral surface of the stator 015. A feeder 017 feeds current to the windings of the stator 015 and is connected to a flexible printed circuit board.
The bearing means 04 is a compound ball bearing, and an inner ring 06 thereof is fixed to the outer surface of the fixed shaft 010, and an outer ring 05 thereof is fixed to the inner peripheral surface of an inner circumferential wall 032 of the rotor hub 03. A part of the inner ring 06 can be formed integrally with the fixed shaft 010 according to circumstances, as shown in FIG. 9; and the outer ring 05 can be formed integrally with the whole structure of the compound ball bearing in certain cases, as shown in the same figure.
The rotary shaft type is also provided with, as shown in FIG. 10, the base (flange) 02, the rotor hub (hub member) 03 that rotates relative to the base, a rotary shaft 020 that is vertically installed on the rotor hub 03, and the bearing means 04 interposed between the rotary shaft 020 and the base 02. The recording medium such as a magnetic disk (not shown) is mounted on the. rotor hub 03. The stator 015 is installed on the outer peripheral surface of the inner cylindrical wall 014 of the base 02, and the permanent magnet 016 is installed on the inner peripheral surface of the outer circumferential wall 013 of the rotor hub 03 so as to face the outer peripheral surface of the stator 015. The symbol 017 denotes the feeder for feeding current to the windings of the stator 015, which is connected to a flexible printed circuit board.
The bearing means 04 is a compound ball bearing, and the inner ring 06 thereof is fixed to the outside to the rotary shaft 020, and the outer ring 05 thereof is fixed to the inner peripheral surface of the cylindrical wall 014 of the base 02. A part of the inner ring 06 can be formed integrally with the rotary shaft 020 according to circumstances, as shown in FIG. 10; and the outer ring 05 can be formed integral with the whole structure of the compound ball bearing in certain cases, as shown in the same figure.
In a certain case, the rotor hub 03 and the rotary shaft 020 each manufactured separately can be assembled into a unit, as shown in FIG. 10; and in another case, they can be manufactured as an integral unit from the beginning. In the latter case, a part of the inner ring 06 cannot be formed integrally with the rotary shaft 020.
In any type of the spindle motor 01, the rotor hub 03 thereof is supported on the base 02 to freely rotate through the compound ball bearing 04 as a rolling bearing interposed between the base 02 and the rotor hub 03. And, the inner ring 06 of the compound ball bearing 04 is fixed to the outside of the fixed shaft 010 vertically installed on the base 02 or to the rotary shaft 020 vertically installed on the rotor hub 03. The outer ring 05 thereof is fixed to the inner peripheral surface of the inner circumferential wall 032 of the rotor hub 03 or to the inner peripheral surface of the inner cylindrical wall 014 of the base 02.
Now, recent demands of the hard disk drive show a remarkable tendency toward increase in the recording capacity, to enhance the impact resistance, to lower the noises, to increase the data access speed, and so forth. In order to answer these requirements, the roller bearing of a spindle motor has gone through improvements in the material composition, enhancements of the precision of the inner and outer rings and rolling elements, etc.
However, when the inner and outer rings and the balls (rolling elements) are made of steel such as bearing steel, metal-to-metal contact occurs between the rolling surfaces of the inner and outer rings and the surfaces of the balls, which contact effects galling and wear to deteriorate the acoustic characteristic, leading to a problem in the acoustic life (recently, the life of the spindle motor is evaluated not by the fatigue life, but by the acoustic life). Further, fretting corrosions (impressions, dilapidated surfaces) form on the rolling surfaces due to shocks and vibrations during transportation, which also deteriorates the acoustic life and the precision of rotation.
Especially in recent years, the rotational speed of a spindle motor is increased to higher than 7200 rpm, and the sound of rotation of the motor becomes increased to that degree, which tends to shorten the acoustic life. Also, in the future, a need for still further increase of the recording capacity is estimated in view of the demand for recording video images and so forth. In order to answer such demands and estimated future problems, the foregoing improvements in the material composition and enhancements of the working precision and the like will not be sufficient.
In recent years, ball materials have been tested and examined which exceed in the non-agglutination property and in wear resistance, and nitride silicon ceramics have been adopted as the rolling element material. There have been discussions about the limitation of the rolling bearing itself, including the ceramic ball bearing made of such new materials, and employment of a fluid bearing has been suggested as a solution to these problems.
FIG. 11 illustrates a rotary shaft type spindle motor 01 with such a fluid bearing. This spindle motor 01 is provided with a base (flange) 02, a rotor hub (hub member) 03 that rotates relative to the base 02, a rotary shaft 020 that is vertically installed on the rotor hub 03, and a fluid bearing 030 interposed between the rotary shaft 020 and the base 02.
A sleeve 031 of the fluid bearing 030 sheathes the rotary shaft 020, and is fixed to the inner peripheral surface of the inner cylindrical wall 014 of the base 02. Lubricating oil is supplied into the sliding area between the sleeve 031 and the rotary shaft 020, and herringbones ( less than -shaped grooves) 033 formed on the circumferential surface of the rotary shaft 020 raise the pressure of the lubricating oil, with the rotation of the rotary shaft 020, which floats the rotary shaft 020 up in the sleeve 031.
Although not detailed in the drawing, similar herringbones are formed on an edge surface of a thrust ring 034 fixed to a lower part of the rotary shaft 020, and lubricating oil is supplied into a gap between the edge surface and an inner surface of a counter plate 037 fixed to the lower end of the sleeve 031. As the rotary shaft 020 turns, the herringbones raise the pressure of the lubricating oil, which makes the counter plate 037 receive the thrust that acts on the rotary shaft 020.
Therefore, the base 02 supports the rotary shaft 020 of the rotor hub 03 for free rotation through the fluid bearing 030 interposed therebetween. The other structure of the motor is basically identical to the spindle motor having the compound ball bearing.
On the other hand, in the fixed shaft type spindle motor with a fluid bearing, which is not illustrated, the sleeve 031 of the fluid bearing 030 is fit to an inner peripheral surface of a wall formed on the rotor hub 03, and a fixed shaft is installed upright on the base 02 and sheathed with the sleeve 031. Therefore, the fixed shaft supports the rotor hub 03 to allow free rotation through the fluid bearing 030 interposed therebetween.
Regardless of whether a ball bearing or a fluid bearing is used, and regardless of whether the spindle motor is a fixed shaft type or a rotary shaft type, the installation of the bearing in the spindle motor is carried out by one of the following methods: press-fitting to the counterpart (rotating components and fixed components), adhesion by adhesives, and press-fit adhesion using both of these.
In case of the press fitting method, the precision of the shape (circularity, cylindricality, surface roughness) of the inner or outer peripheral surface of the counterpart influences the shape from the outer peripheral surface of the outer ring and the inner peripheral surface of the inner ring of the rolling bearing which influence is transferred to the rolling surfaces of the inner and outer rings, i.e., deformation of the rolling surfaces of the inner and outer rings. Also, the external stress caused by press fitting propagates through the outer peripheral surface of the outer ring or through the inner peripheral surface of the inner ring, and produces permanent deformations on the rolling surfaces of the inner and outer rings through the rolling elements to form impressions thereon, which reduces the reliability of the run-out accuracy/NRRO, and the acoustic life, etc., of the motor. In the fluid bearing, the clearance between the sleeve and the shaft varies, which varies the rigidity.
In using an adhesive, stress is produced when the adhesive hardens, which deforms the bearing, also damaging the reliability of the run-out accuracy, increasing noise, and reducing the acoustic life of the motor, and so forth. Further, in the rotary shaft type spindle motor, the assembly of the stator 015 on the outer peripheral surface of the cylindrical wall 014 of the base 02 reduces the accuracy of the inner diameter of the cylindrical wall 014, which, in turn, reduces the bearing precision.
Further, in case of the foregoing press-fitting, adhesion, and press-fit adhesion methods for mounting the bearing, an adhesion groove (refer to adhesion groove 040 in FIG. 9, adhesion groove 041 in FIG. 10) for introducing the adhesive and a run-off groove are needed on the bearing mounting surface on the side of the counterpart, which require additional time (man-hours), leading to a cost increase.
The present invention has been made in view of the foregoing problems, and it is an object of the invention to provide a spindle motor and a bearing assembly that resolve the foregoing problems of the conventional spindle motor, and that remove adverse influences of stress on the precision of the rolling surfaces of the inner and outer rings through the outer peripheral surface of the outer ring and the inner peripheral surface of the inner ring of the bearing. In other words, the object is to reduce the effect of stress on shape precision (circularity, cylindricality, surface roughness) of the inner or outer peripheral surface of the counterpart, which stress is created in mounting the bearing by press-fitting, adhesion, or press-fit adhesion, and to thereby enhance the reliability of the run-out accuracy/NRRO, reduce the noise, and prolong the acoustic life, etc., of the spindle motor, and to reduce the manufacturing cost thereof.
According to the first aspect of the invention, the spindle motor is a fixed shaft type spindle motor in which a fixed shaft is vertically supported on a base and a rotor hub is supported for free rotation by the fixed shaft through a bearing which is a compound ball bearing, with a larger diameter portion of a connection member fixed within an end of an outer ring of the compound ball bearing, and a smaller diameter portion of the connection member fastened to the rotor hub. Thus, the outer ring of the compound ball bearing is fastened to the rotor hub through the connection member.
As a result, the rotor hub (the component on the rotating side) being one of the two counterparts (a component on the rotating side and a component on the fixing side) that have the compound ball bearing mounted therebetween can omit the inner peripheral wall which has conventionally been regarded as necessary for fitting the outer ring of the compound ball bearing thereto. Therefore, the stress otherwise resulting from the shape precision (circularity, cylindricality, surface roughness) of the inner peripheral surface of the wall, and the stress caused by the press-fitting, adhesion, or press-fit adhesion mounting of the bearing are eliminated. Accordingly, adverse influences on the precision of the rolling surfaces of the inner and outer rings through the outer peripheral surface of the outer ring of the bearing are eliminated, thereby enhancing the reliability of the run-out accuracy/NRRO, the noises, and the acoustic life, etc., of the spindle motor.
Further, since the rotor hub can be configured without the inner peripheral wall, which has conventionally been regarded as necessary for fitting the outer ring of the compound ball bearing thereto, the adhesion groove (the groove for filling adhesives) and the run-off groove that are conventionally formed on the inner peripheral surface of the wall become unnecessary, thereby reducing the man-hours and manufacturing cost.
According to the second aspect of the invention, the compound ball bearing is substituted with a fluid bearing, with the larger diameter portion of the connection member fixed within an end of a sleeve of the fluid bearing, and the smaller diameter portion of the connection member fastened to the rotor hub. Thus, the sleeve of the fluid bearing is fastened to the rotor hub through the connection member.
Also with use of a fluid bearing, the inner peripheral wall, which has conventionally been regarded as necessary for fitting the sleeve of the fluid bearing thereto, may be eliminated, along with the aforementioned stresses. Accordingly, adverse influences on the precision of the sliding surfaces of the sleeve and the fixed shaft and on the clearance therebetween are eliminated, thereby enhancing the reliability of the run-out accuracy/NRRO, the noises, the acoustic life, and the rigidity, etc., of the spindle motor.
Further, since the inner peripheral wall, which has conventionally been regarded as necessary for fitting the sleeve of the fluid bearing thereto, is omitted, the adhesion groove and the run-off groove that are conventionally formed on the inner peripheral surface of the wall become unnecessary, thereby reducing the man-hours and the manufacturing cost.
According to a third aspect of the invention, there is provided a rotary shaft type spindle motor in which a rotary shaft is fixed to a rotor hub and the rotary shaft is supported for freely rotating through a bearing, wherein the bearing is a compound ball bearing. A larger diameter portion of a stepped connection member is fixed within an end of an outer ring of the compound ball bearing, and a smaller diameter portion of the connection member is fixed to the base. Thus, the outer ring of the compound ball bearing is fastened to the base through the connection member.
As a result, the base (the component on the fixed side) being the one of the two counterparts (the component on the rotating side and the component on the fixed side) that mount the compound ball bearing therebetween can be made without the inner peripheral wall, which has conventionally been regarded as necessary to fit the outer ring of the compound ball bearing thereto. Therefore, the stress resulting from the shape precision (circularity, cylindricality, surface roughness) of the inner peripheral surface of the wall and the stress caused by the press-fitting, adhesion, or press-fit adhesion method of mounting the bearing are eliminated. Accordingly, adverse influences on the precision of the rolling surfaces of the inner and outer rings through the outer peripheral surface of the outer ring of the bearing are eliminated, thereby enhancing the reliability of the run-out accuracy/NRRO, the noises, and the acoustic life, etc., of the spindle motor.
Further, since the inner peripheral wall, which has conventionally been regarded as necessary for fitting the outer ring of the compound ball bearing thereto, is eliminated, the adhesion groove and the run-off groove that are conventionally formed on the inner peripheral surface of the wall become unnecessary, thereby reducing the man-hours and the manufacturing cost.
In a fourth aspect of the invention, the compound ball bearing of the third aspect is replaced with a fluid bearing, with a larger diameter portion of the stepped connection member fixed within an end of a sleeve of the fluid bearing, and the smaller diameter portion of the connection member fastened to the base. Thus, the sleeve of the fluid bearing is fastened to the base through the connection member.
As a result, in the fourth aspect also, the aforementioned stresses are eliminated and, accordingly, adverse influences on the precision of the sliding surfaces of the sleeve and the fixed shaft and on the clearance between the sliding surfaces of the two are eliminated, thereby enhancing the reliability of the run-out accuracy/NRRO, the noise, the acoustic life, and the rigidity, etc., of the spindle motor.
Further, in the fourth aspect also, because the inner peripheral wall, which has conventionally been regarded as necessary for fitting the sleeve of the fluid bearing thereto, is eliminated, the adhesion groove and the run-off groove can also be eliminated, thereby reducing the man-hours and lowering the manufacturing cost.